1. Field of the Invention
The present invention relates to a flexible optical interconnection structure and a method for fabricating the same, more particularly, to a flexible optical interconnection structure and a method for fabricating the same with improved mechanical reliability.
2. Related Art
With the expansion of services and applications for handling a large capacity of data such as images in the electronic equipment such as personal computers, cellular phones and television sets, the developments of high-speed and large-capacity data communication technologies are being promoted. In this technical environment, optical interconnection attracts the attention for enabling high-speed and large-capacity data communication within the electronic equipment or among a set of electronic equipments as recited above, as well as within an electronic circuit board or between a plurality of the electronic circuit boards.
Conventionally, electric interconnections (wirings) have been used for connection of signal, transmission line among the electronic circuit boards. Particularly for parts requiring the flexibility within the electronic equipment or among the electronic circuit boards, the use of electric interconnections using a Flexible Printed Circuit (FPC) with multiple cores and flexibility or a thin coaxial cable has been researched.
However, problems such as crosstalk, electromagnetic interference, band limitation, loss due to high frequency in signal transmission using the electric interconnections appear in accordance with speedup of the signal transmission. The transmission capacitance per one channel (one core) for the electric interconnection is at most several Gbps, which requires additional channels or cores and waveform correction circuits in order to increase the signal speed in the future. However, as the number of cores increases, the interconnection cost may increase as well as the interconnection volume increases, which may suffer a wiring space shortage. In order to speedup the signal transmission, it is required to implement the waveform correction circuit for correcting the waveform turbulence near a terminal end of the electric interconnection caused by the signal delay, etc. in the electric interconnection. Additional waveform correction circuit may increase the overall cost for the electric interconnection because the device cost for the waveform correction circuits itself and its mount cost may be added. In addition, the electric interconnection behaves as an antenna by itself and may irradiate electromagnetic waves outside and may generate electromagnetic noises due to incoming electromagnetic waves.
A high-speed transmission of greater than several Gbps, which is irrealizable in the electric interconnection without causing the problems such as crosstalk, electromagnetic interference, can be realized by optical interconnection using a light (optical signal) in place of the electric signal as a transmission medium. The optical interconnection for transmitting signals faster than the electric interconnection is mainly used for the long-distance information transmission, for example, intercontinental and inter-city networks which use optical interconnections for single mode transmission with less delay in the optical pulse. As for the short-distance LAN (Local Area Network), from the view point of easiness in connectivity of optical interconnections, multi mode transmission is mainly used, since the optical connection between the equipments is easy because of a large core diameter. Such high-speed transmission is realized by using the optical fiber, and a single mode fiber or a multimode fiber corresponding to respective transmission modes is used.
On the other hand, as to the interconnection within the equipment, among the electric circuit boards, and within the electric circuit board, the optical transmission with the use of an optical waveguide has been researched. For realizing such optical transmission, numerous multimode optical waveguides mainly made of a polymer have been developed. In each of these optical waveguides, a photoelectric conversion element for converting an electrical signal into an optical signal and vice versa is provided, and the electrical signal is converted to the optical signal and input to the optical waveguide, and the optical signal transmitted through the optical waveguide is converted again to the electrical signal. A light emitting element and a driver for driving the light emitting element are used for converting the electrical signal into the optical signal. A light receiving element and an amplifier for amplifying a received signal are used for converting the optical signal into the electrical signal.
The light emitting element, the light receiving element, the driver, and the amplifier are surface-mounted on the optical interconnection structure, in which an emitting direction of a light emitted from the light emitting element and a longitudinal direction of the optical waveguide (orientation of the light spread in the optical waveguide) are arranged to be perpendicular to each other. In addition, the longitudinal direction of the optical waveguide and a direction of a light output from the optical waveguide and incident to the light receiving element are arranged to be perpendicular to each other. Accordingly, it is necessary to provide an optical path conversion part for converting (changing) a direction of an optical path by 90° in a light input part from the light emitting element to the optical waveguide and a light output part from the optical waveguide to the light receiving element. So as to realize the optical path conversion parts, following technique is used. Namely, each of the light input part and the light output part of a core of the optical waveguide is provided with a surface with an angle of 45° (45° surface) by processing, and the light is reflected at an angle of 90° by the 45° surface as a mirror surface.
In the case that the signal is transmitted through the optical interconnection within the equipment having a movable element or among the electric circuit boards, the optical interconnection structure should be sustainable for bending or deformation. Therefore, a flexible optical interconnection structure is lively developed. For realizing the signal transmission in a hinge part or a movable device of a portable telephone, a personal computer, and the like, the optical interconnection structure with the use of polymer waveguide is mainly developed.
When the signal is transmitted through the flexible optical interconnection structure using the polymer waveguide within the equipment having the movable element or among the electric circuit boards, the flexible optical interconnection structure should be sustainable for repeated bending or deformation. Japanese Patent Laid-Open No. 2006-323316 (JP-A-2006-323316) discloses a flexible optical interconnection structure using a polymer which is excellent in bending property such as polyimide or norbornene.
As to the use of the flexible optical interconnection structure, there are two cases, namely, the flexible optical interconnection structure is used as an interconnection consisted of the optical waveguide, and the optical waveguide is laminated together with other parts and electrical interconnection (wiring). In the latter case, a metal wiring for electric power transmission or electrical signal transmission, a base material film for making the metal wiring, and a backing plate for improving a strength of the flexible optical interconnection structure and for preventing the optical interconnection structure from damages due to abrasion or breakage are used in addition to the optical waveguide.
When the flexible optical interconnection structure is not symmetrical with respect to a center plane in its thickness (depthwise) direction, a neutral surface (a virtual surface that is not affected by the expansion and contraction by bending) is shifted from a center position in the thickness direction, since these various parts comprise various materials in which elastic modulus and thickness thereof are different from each other. When the flexible optical interconnection structure is bent, a tensile stress is applied to an outer part of a bent part (namely, a part provided outside with respect to the neutral surface) so that the material is expanded, while a compressive stress is applied to an inner part of the bent part (namely, a part provided inside with respect to the neutral surface) so that the material is contracted. A part without a strain due to the expansion and contraction is the neutral surface.
In the polymer waveguide, a rigid structure such as benzene ring is usually introduced into the core, so as to increase a refractive index of the core compared with that of a clad. Therefore, the material of the core has an elastic modulus greater than that of the clad, namely, the core comprises a hard and fragile material compared with the clad. The material having a large elastic modulus is disadvantageous in improving the repeated bending property, so that it is preferable that the tensile stress and the compressive stress applied to the polymer waveguide are as small as possible. Therefore, it is required that the core having the large elastic modulus is positioned at the neutral surface or in vicinity of the neutral surface in the flexible optical interconnection structure.
For reducing the tensile stress and the compressive stress to be applied to the flexible optical interconnection structure, there is a technique of reducing the overall thickness of the flexible optical interconnection structure. However, there is a restriction in a lower limit of the thickness of the flexible optical interconnection structure, when the aforementioned elements such as metal wiring, backing plate and the like are added, as described in Japanese Patent Laid-Open No. 2006-339173 (JP-A-2006-339173).
The flexible optical interconnection structure to be used in the movable part within the equipment and between the circuit boards should be excellent in the repeated bending property. If not, a malfunction such as breakage often occurs in the core part that is particularly inferior in the bending property in the flexible optical interconnection structure.